Power amplifiers (PAs) are indispensable components in communication systems and are nonlinear in nature. At the PA output, the nonlinearity not only causes spectral regrowth, which interferes with adjacent channel signals, but also degrades the bit error rate of the inband signal. To compensate for the nonlinearity, PA linearization is often necessary.
Among the linearization techniques, digital baseband predistortion is highly cost effective. It adds a functional block, called a predistorter, before the digital-to-analog conversion and RF upconversion but after any other baseband processing. The predistorter ideally applies the exact inverse response of the power amplifier to a scaled version of the input signal. Therefore, the output of the predistorter-PA cascade is the input signal multiplied by a scaling factor. To construct such a predistorter, a feedback path is needed to capture the output of the PA.
The performance of baseband predistortion relies on accurate PA modeling and obtaining a precise inverse of the PA. In reality, however, the performance can also be affected significantly by imperfections in the upconverter in the transmitter and the downconverter in the feedback path. These imperfections are caused by the analog components employed in both the upconverter and the downconverter, such as mixers, filters, quadrature modulators, and quadrature demodulators.
To reduce these impairments in the downconverter, filters that have relatively flat frequency may be used, such as LC bandpass and lowpass filters, and digital demodulation, which is free of any demodulation errors. With these configurations and careful design, the imperfections in the downconverter can be negligible.
In the upconverter, there are two common configurations, but the analog impairments normally cannot be neglected in either. The first uses digital modulation and two-stage upconversion, i.e., first upconvert baseband signals to IF and then to RF. Because of the stringent image rejection requirements of the transmitter, a SAW filter is usually used in the IF stage for this configuration. But the SAW filter often has large frequency response variations, and therefore distorts the predistorted signal. The second choice for the upconverter is to use direct upconversion, in which the I/Q data streams are directly modulated to RF. This structure enables the upconverter to be easily reconfigured to generate RF signals in different frequency bands. It also uses fewer components and is easier to integrate. However, in practice, the quadrature carriers in the analog modulator do not have exactly the same amplitudes and an exact phase difference of 90 degrees. These effects are called gain/phase imbalance and cause cross-talk between the I and Q channels. In addition, leakage of the carriers to the transmitted signal manifests itself in the demodulated received signal as a dc offset. If uncompensated, the gain/phase imbalance and the dc offset would have a large effect on predistortion performance. Various techniques have been proposed in the past to compensate for these impairments. In these techniques, the gain/phase imbalance are assumed to be frequency independent. However, experiments performed by the inventors indicate that the gain/phase imbalance exhibits frequency dependent behavior when the signal bandwidth becomes wide, for example, 15 MHz.